It has been found that medicaments for administration by inhalation should be of a controlled particle size in order to achieve maximum penetration into the lungs, preferably in the range of 1 to 10 micrometers in diameter. Unfortunately, powders in this particle size range, hereinafter referred to as fine powders, for example micronised powders, usually have very poor flow characteristics due to the cohesive forces between the individual particles which make them readily agglomerate together to form bridges which are not readily broken apart to become free flowing. These characteristics create handling and metering difficulties which adversely affect the accurate dispensing of doses of the powder.
An apparatus for supplying particles in fine dust form in measured doses is known from DE 3607187. Powder is supplied from a hopper to a vibrator with an outlet. The hopper uses an agitator and compressed air to maintain the powder in a dry and deagglomerated form. The vibrator/outlet unit allows an approximate control of flow of the powder onto a rotating metering plate underneath, provided with a ring of pockets in its upper surface, the pockets being filled with powder as they pass underneath the outlet. As the metering plate rotates, a doctor blade wipes excess powder from the upper surface, and the doses of powder in the pockets are removed and supplied to a processing station by means of a suction tube.
Another apparatus for accurately dispensing programmed weights of particulate solids which tend to agglomerate is known from U.S. Pat. No. 4,688,610. Again, powder is supplied from a hopper with an agitator to a vibrating conveyor and on to a discharge area. A microprocessor controller is used to repeatedly read the weight of solids on the discharge area and accordingly control the activation of the agitator and vibrator to dispense precisely weighed quantities of the particulate solids.
It has been found that by careful sizing of fine agglomerated powder it is possible to make use of the cohesive forces between the particles to create agglomerates of powder which are free flowing. However, such agglomerates are easily destroyed by physical contact with other bodies, though exposure to vibration does not adversely affect them. Careful handling is therefore required to take advantage of the free flow characteristics.